"We've gained a player who can do the unexpected and is extremely creative. Hakan should and will give our team an important boost in quality."

The player's hopes of a move appeared to stall when financially-troubled Hamburg vetoed his sale to a Bundesliga rival, but their move for Pierre-Michel Lasogga from Hertha Berlin meant funds were needed.

Calhanoglu said that meant he could "take the next step forward," adding: "Bayer have a great team and they've been in Europe regularly in recent years.

"To reach the Champions League group stages and compete at the top of the Bundesliga is a big challenge for me. I'm really looking forward to it and I'll give everything to help the team."